wdiff rfc9276.original rfc9276.txt

Network Working Group

Internet Engineering Task Force (IETF) W. Hardaker
Internet-Draft
Request for Comments: 9276 USC/ISI
Updates: 5155 (if approved)
BCP: 236 V. Dukhovni
Intended status:
Updates: 5155 Bloomberg, L.P.
Category: Best Current Practice Bloomberg, L.P.
Expires: 26 November 2022 25 May August 2022
ISSN: 2070-1721

Guidance for NSEC3 parameter settings
draft-ietf-dnsop-nsec3-guidance-10 Parameter Settings

Abstract

NSEC3 is a DNSSEC mechanism providing proof of non-existence nonexistence by
asserting that there are no names that exist between two domain names
within a zone. Unlike its counterpart NSEC, NSEC3 avoids directly
disclosing the bounding domain name pairs. This document provides
guidance on setting NSEC3 parameters based on recent operational
deployment experience. This document updates [RFC5155] RFC 5155 with guidance
about selecting NSEC3 iteration and salt parameters.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working memo documents an Internet Best Current Practice.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
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Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
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This Internet-Draft will expire on 26 November 2022.
https://www.rfc-editor.org/info/rfc9276.

This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. NSEC3 Parameter Value Discussions . . . . . . . . . . . . . . 3
2.1. Algorithms . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Iterations . . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Salt . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Recommendations for Deploying and Validating NSEC3 Records . 6
3.1. Best-practice Best Practice for Zone Publishers . . . . . . . . . . . . 6
3.2. Recommendation for Validating Resolvers . . . . . . . . . 7
3.3. Recommendation for Primary / and Secondary Relationships . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Operational Considerations . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Deployment measurements Measurements at time Time of publication . . . 10 Publication
Appendix B. Computational burdens Burdens of processing Processing NSEC3
iterations . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix C. Iterations
Acknowledgments . . . . . . . . . . . . . . . . . . 10
Appendix D. GitHub Version of This Document . . . . . . . . . . 11
Appendix E. Implementation Notes . . . . . . . . . . . . . . . . 11
E.1. OpenDNSSEC . . . . . . . . . . . . . . . . . . . . . . . 11
E.2. PowerDNS . . . . . . . . . . . . . . . . . . . . . . . . 11
E.3. Knot DNS and Knot Resolver . . . . . . . . . . . . . . . 11
E.4. Google Public DNS Resolver . . . . . . . . . . . . . . . 12
E.5. Google Cloud DNS . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12

1. Introduction

As with NSEC [RFC4035], NSEC3 [RFC5155] provides proof of non-
existence
nonexistence that consists of signed DNS records establishing the non-
existence
nonexistence of a given name or associated Resource Record Type
(RRTYPE) in a DNSSEC [RFC4035] signed zone. In DNSSEC-signed zone [RFC4035]. However, in the case of
NSEC3, however, the names of valid nodes in the zone are obfuscated through
(possibly multiple iterations of) hashing (currently only SHA-1 is in
use on the Internet).

NSEC3 also provides "opt-out support", allowing for blocks of
unsigned delegations to be covered by a single NSEC3 record. Use of
the opt-out feature allows large registries to only sign as many
NSEC3 records as there are signed DS or other RRsets Resource Record sets
(RRsets) in the zone; with opt-out, unsigned delegations don't
require additional NSEC3 records. This sacrifices the tamper-resistance tamper-
resistance of the proof of non-
existence nonexistence offered by NSEC3 in order to
reduce memory and CPU overheads.

NSEC3 records have a number of tunable parameters that are specified
via an NSEC3PARAM record at the zone apex. These parameters are the
hash algorithm, the processing flags, the number of hash iterations iterations,
and the salt. Each of these has security and operational
considerations that impact both zone owners and validating resolvers.
This document provides some best-practice recommendations for setting
the NSEC3 parameters.

1.1. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. NSEC3 Parameter Value Discussions

The following sections describes describe the background of the parameters for
the NSEC3 and NSEC3PARAM resource record types. RRTYPEs.

2.1. Algorithms

The algorithm field is not discussed by this document. Readers are
encouraged to read [RFC8624] for guidance about DNSSEC algorithm
usage.

2.2. Flags

The NSEC3PARAM flags field currently contains only reserved and
unassigned flags. Individual However, individual NSEC3 records, however, records contain the
"Opt-Out" flag [RFC5155], which [RFC5155] that specifies whether that NSEC3 record
provides proof of non-existence. nonexistence. In general, NSEC3 with the Opt-Out
flag enabled should only be used in large, highly dynamic zones with
a small percentage of signed delegations. Operationally, this allows
for fewer signature creations when new delegations are inserted into
a zone. This is typically only necessary for extremely large
registration points providing zone updates faster than real-time
signing allows or when using memory-constrained hardware. Operators
considering the use of NSEC3 are advised to fully test their zones
deployment architectures and authoritative servers under both regular
operational loads to determine carefully weigh the tradeoffs using NSEC3 instead costs
and benefits of choosing NSEC3 over NSEC. Smaller zones, or large
but relatively static zones, are encouraged to not use a the opt-opt
flag and to take advantage of DNSSEC's proof-of-non-existence support. authenticated denial of
existence.

2.3. Iterations

NSEC3 records are created by first hashing the input domain and then
repeating that hashing using the same algorithm a number of times
based on the iteration parameter in the NSEC3PARM NSEC3PARAM and NSEC3 records.
The first hash with NSEC3 is typically sufficient to discourage zone
enumeration performed by "zone walking" an unhashed NSEC chain.

Note that [RFC5155] describes the Iterations field to be "The as follows

| The Iterations field defines the number of additional times the
| hash function has been performed." performed.

This means that an NSEC3 record with an Iterations field of 0
actually requires one hash iteration.

Only determined parties with significant resources are likely to try
and uncover hashed values, regardless of the number of additional
iterations performed. If an adversary really wants to expend
significant CPU resources to mount an offline dictionary attack on a
zone's NSEC3 chain, they'll likely be able to find most of the
"guessable" names despite any level of additional hashing iterations.

Most names published in the DNS are rarely secret or unpredictable.
They are published to be memorable, used and consumed by humans.
They are often recorded in many other network logs such as email
logs, certificate transparency logs, web page links, intrusion intrusion-
detection systems, malware scanners, email archives, etc. Many times
a simple dictionary of commonly used domain names prefixes (www,
mail, imap, login, database, etc.) can be used to quickly reveal a
large number of labels within a zone. Because of this, there are
increasing performance costs yet diminishing returns associated with
applying additional hash iterations beyond the first.

Although Section 10.3 of [RFC5155] specifies the upper bounds for the
number of hash iterations to use, there is no published guidance for
zone owners about good values to select. Recent academic studies
have shown that NSEC3 hashing provides only moderate protection
[GPUNSEC3][ZONEENUM].
[GPUNSEC3] [ZONEENUM].

2.4. Salt

NSEC3 records provide an additional salt value, which can be combined
with an FQDN a Fully Qualified Domain Name (FQDN) to influence the resulting
hash, but properties of this extra salt are complicated.

In cryptography, salts generally add a layer of protection against
offline, stored dictionary attacks by combining the value to be
hashed with a unique "salt" value. This prevents adversaries from
building up and remembering a single dictionary of values that can
translate a hash output back to the value that it was derived from.

In the case of DNS, the situation is different because the hashed
names placed in NSEC3 records are always implicitly "salted" by
hashing the fully-qualified domain name FQDN from each zone. Thus, no single pre-computed table
works to speed up dictionary attacks against multiple target zones.
An attacker is always required to compute a complete dictionary per
zone, which is expensive in both storage and CPU time.

To understand the role of the additional NSEC3 salt field, we have to
consider how a typical zone walking attack works. Typically, the
attack has two phases - phases: online and offline. In the online phase, an
attacker "walks the zone" by enumerating (almost) all hashes listed
in NSEC3 records and storing them for the offline phase. Then, in
the offline cracking phase, the attacker attempts to crack the
underlying hash. In this phase, the additional salt value raises the
cost of the attack only if the salt value changes during the online
phase of the attack. In other words, an additional, constant salt
value does not change the cost of the attack.

Changing a zone's salt value requires the construction of a complete
new NSEC3 chain. This is true both when re-signing the entire zone
at once, once and when incrementally signing it in the background where the
new salt is only activated once every name in the chain has been
completed. As a result, re-salting is a very complex operation, with
significant CPU time, memory, and bandwidth consumption. This makes
very frequent re-salting impractical, impractical and renders the additional salt
field functionally useless.

3. Recommendations for Deploying and Validating NSEC3 Records

The following subsections describe recommendations for the different
operating realms within the DNS.

3.1. Best-practice Best Practice for Zone Publishers

First, if the operational or security features of NSEC3 are not
needed, then NSEC SHOULD be used in preference to NSEC3. NSEC3
requires greater computational power (see Appendix B) for both
authoritative servers and validating clients. Specifically, there is
a nontrivial complexity in finding matching NSEC3 records to randomly
generated prefixes within a DNS zone. NSEC mitigates this concern.
If NSEC3 must be used, then an iterations count of 0 MUST be used to
alleviate computational burdens. Note that extra iteration counts
other than 0 increase the impact of CPU-exhausting DoS attacks, and
also increase the risk of interoperability problems.

Note that deploying NSEC with minimally covering NSEC records
[RFC4470] also incurs a cost, and zone owners should measure the
computational difference in deploying either RFC4470 [RFC4470] or NSEC3.

In short, for all zones, the recommended NSEC3 parameters are as
shown below:

; SHA-1, no extra iterations, empty salt:
;
bcp.example. IN NSEC3PARAM 1 0 0 -

For small zones, the use of opt-out based opt-out-based NSEC3 records is NOT
RECOMMENDED.

For very large and sparsely signed zones, where the majority of the
records are insecure delegations, opt-out MAY be used.

Operators SHOULD NOT use a salt by indicating a zero-length salt
value instead (represented as a "-" in the presentation format).

If salts are used, note that since the NSEC3PARAM RR is not used by
validating resolvers (see [RFC5155] section 4), Section 4 of [RFC5155]), the iterations and
salt parameters can be changed without the need to wait for RRsets to
expire from caches. A complete new NSEC3 chain needs to be
constructed and the full zone needs to be re-signed.

3.2. Recommendation for Validating Resolvers

Because there has been a large growth of open (public) DNSSEC
validating resolvers that are subject to compute resource constraints
when handling requests from anonymous clients, this document
recommends that validating resolvers change reduce their behavior with
respect to large iteration values. count
limits over time. Specifically, validating resolver operators and
validating resolver software implementers are encouraged to continue
evaluating NSEC3 iteration count deployments
but deployment trends and lower their default
acceptable iteration limits over time. Similarly,
because Because treating a high
iterations count as insecure leaves zones subject to attack,
validating resolver operators and validating resolver software
implementers are further encouraged to lower their default and acceptable limit for
returning SERVFAIL when processing NSEC3 parameters containing large
iteration count values. See Appendix A for measurements taken near
the time of publication of this document and potential starting
points.

Validating resolvers MAY return an insecure response to their clients
when processing NSEC3 records with iterations larger than 0. Note
also that a validating resolver returning an insecure response MUST
still validate the signature over the NSEC3 record to ensure the
iteration count was not altered since record publication (see
[RFC5155] section 10.3).
Section 10.3 of [RFC5155]).

Validating resolvers MAY also return a SERVFAIL response when
processing NSEC3 records with iterations larger than 0. Validating
resolvers MAY choose to ignore authoritative server responses with
iteration counts greater than 0, which will likely result in
returning a SERVFAIL to the client when no acceptable responses are
received from authoritative servers.

Validating resolvers returning an insecure or SERVFAIL answer to
their client after receiving and validating an unsupported NSEC3
parameter from the authoritative server(s) SHOULD return an Extended
DNS Error (EDE) [RFC8914] EDNS0 option of value (RFC EDITOR: TBD). 27. Validating
resolvers that choose to ignore a response with an unsupported
iteration count (and that do not validate the signature) MUST NOT
return this EDE option.

Note that this specification updates [RFC5155] by significantly
decreasing the requirements originally specified in Section 10.3 of
[RFC5155]. See the Security Considerations (Section 4) for arguments
on how to handle responses with non-zero iteration count.

3.3. Recommendation for Primary / and Secondary Relationships

Primary and secondary authoritative servers for a zone that are not
being run by the same operational staff and/or using the same
software and configuration must take into account the potential
differences in NSEC3 iteration support.

Operators of secondary services should advertise the parameter limits
that their servers support. Correspondingly, operators of primary
servers need to ensure that their secondaries support the NSEC3
parameters they expect to use in their zones. To ensure reliability,
after primaries change their iteration counts, they should query
their secondaries with known non-existent nonexistent labels to verify the
secondary servers are responding as expected.

4. Security Considerations

This entire document discusses security considerations with various
parameters
parameter selections of NSEC3 and NSEC3PARAM fields.

The point where a validating resolver returns insecure vs versus the
point where it returns SERVFAIL must be considered carefully.
Specifically, when a validating resolver treats a zone as insecure
above a particular value (say 100) and returns SERVFAIL above a
higher point (say 500), it leaves the zone subject to attacker-in-
the-middle attacks as if it was were unsigned between these values.
Thus, validating resolver operators and software implementers SHOULD
set the point above which a zone is treated as insecure for certain
values of NSEC3 iterations counts to the same as the point where a
validating resolver begins returning SERVFAIL.

5. Operational Considerations

This entire document discusses operational considerations with
various parameters parameter selections of NSEC3 and NSEC3PARAM fields.

6. IANA Considerations

This document requests a new allocation

IANA has allocated the following code in the First Come First Served
range [RFC8126] of the "Extended DNS Error Codes" of registry within the
"Domain Name System (DNS) Parameters" registration table with the following
characteristics:

* registry:

INFO-CODE: (RFC EDITOR: TBD)

* 27
Purpose: Unsupported NSEC3 iterations value

*
Reference: (RFC EDITOR: this document) RFC 9276

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.

[RFC4470] Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
and DNSSEC On-line Signing", RFC 4470,
DOI 10.17487/RFC4470, April 2006,
<https://www.rfc-editor.org/info/rfc4470>.

[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<https://www.rfc-editor.org/info/rfc5155>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
Lawrence, "Extended DNS Errors", RFC 8914,
DOI 10.17487/RFC8914, October 2020,
<https://www.rfc-editor.org/info/rfc8914>.

7.2. Informative References

[GPUNSEC3] Wander, M., Schwittmann, L., Boelmann, C., and T. Weis,
"GPU-Based NSEC3 Hash Breaking", DOI 10.1109/NCA.2014.27,
August 2014, <https://doi.org/10.1109/NCA.2014.27>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8624] Wouters, P. and O. Sury, "Algorithm Implementation
Requirements and Usage Guidance for DNSSEC", RFC 8624,
DOI 10.17487/RFC8624, June 2019,
<https://www.rfc-editor.org/info/rfc8624>.

[ZONEENUM] Wang, Z., Xiao, L., and R. Wang, "An efficient DNSSEC zone
enumeration algorithm", n.d.. DOI 10.2495/MIIT130591, April
2014, <https://doi.org/10.2495/MIIT130591>.

Appendix A. Deployment measurements Measurements at time Time of publication Publication

At the time of publication, setting an upper limit of 100 iterations
for treating a zone as insecure is interoperable without significant
problems, but at the same time still enables CPU-exhausting DoS
attacks.

At the time of publication, returning SERVFAIL beyond 500 iterations
appears to be interoperable without significant problems.

Appendix B. Computational burdens Burdens of processing Processing NSEC3 iterations Iterations

The queries per second (QPS) of authoritative servers will decrease
due to computational overhead when processing DNS requests for zones
containing higher NSEC3 iteration counts. The table below shows the
drop in QPS for various iteration counts.

+============+=============================+
| Iterations | QPS [% of 0 iterations Iterations QPS] |
|------------+-----------------------------|
+============+=============================+
| 0 | 100 % 100% |
+------------+-----------------------------+
| 10 | 89 % 89% |
+------------+-----------------------------+
| 20 | 82 % 82% |
+------------+-----------------------------+
| 50 | 64 % 64% |
+------------+-----------------------------+
| 100 | 47 % 47% |
+------------+-----------------------------+
| 150 | 38 % 38% |

Appendix C.
+------------+-----------------------------+

Table 1: Drop in QPS for Various
Iteration Counts

Acknowledgments

The authors would like to thank the dns-operations discussion
participants, participants in the dns-
operations discussion, which took place on mattermost hosted by DNS-OARC. DNS-
OARC.

Additionally, the following people contributed text or review
comments to the draft: this document:

* Vladimir Cunat

* Tony Finch

* Paul Hoffman

* Warren Kumari

* Alexander Mayrhofer

* Matthijs Mekking

* Florian Obser

* Petr Spacek

* Paul Vixie

* Tim Wicinski

Appendix D. GitHub Version of This Document

(RFCEditor: remove this section)

While this document is under development, it can be viewed, tracked,
issued, pushed with PRs, ... here:

https://github.com/hardaker/draft-hardaker-dnsop-nsec3-guidance

Appendix E. Implementation Notes

(RFCEditor: remove this section)

The following implementations have implemented the guidance in this
document. They have graciously provided notes about the details of
their implementation below.

E.1. OpenDNSSEC

The OpenDNSSEC configuration checking utility will alert the user
about nsec3 iteration values larger than 100.

E.2. PowerDNS

PowerDNS 4.5.2 changed the default value of nsec3-max-iterations to
150.

E.3. Knot DNS and Knot Resolver

Knot DNS 3.0.6 warns when signing with more than 20 NSEC3 iterations.
Knot Resolver 5.3.1 treats NSEC3 iterations above 150 as insecure.

E.4. Google Public DNS Resolver

Google Public DNS treats NSEC3 iterations above 100 as insecure since
September 2021.

E.5. Google Cloud DNS

Google Cloud DNS uses 1 iteration and 64-bits of fixed random salt
for all zones using NSEC3. These parameters cannot be adjusted by
users.

Authors' Addresses

Wes Hardaker
USC/ISI
Email: ietf@hardakers.net

Viktor Dukhovni
Bloomberg, L.P.
Email: ietf-dane@dukhovni.org